Process for oxidizing lower alkyl benzenes



States Patent Research Corporation, San Francisco, Calif, a corporation of Delaware No Drawing. Application December 13, 1952, Serial No. 325,895

4 Claims. (Cl. 260-524) This invention relates to a process for oxidizing lower alkyl benzene hydrocarbons to produce derivatives of benzene carboxylic acids and the acids themselves.

The Willgerodt reaction has been known for more. than fifty years. The reaction has been applied to eflect the oxidation of ketones, aldehydes and unsaturated hydrocarbons to acid amides. In the past the reaction has been practiced in batch operation and the results viewed macroscopically indicate that the rate. of reaction islow, the time of reaction is long, and the yields of amide are low, usually less than 60% of theoretical. The literature indicates that the Willgerodt reaction has geen extensively explored and has received a great deal of time and com sideration by skilled workers in the art. So far as is known, no commercial application of. the Willgerodt. reaction has been developed. The poor yields and long reaction times which have been characteristic of past applications of the reaction appear to constitute an economic block to its commercial adaptation.

U. S. Patent No. 2,610,980 describes the extension of alkyl benzene hydrocarbons and exemplifies the oxidation of toluene, ethyl benzene, cumene and para-xylene. The reaction times and yields obtained during the oxida-. tion of these. materials pretty much. parallel those reported in the literature in connection with the oxidation of other materials by the Willgerodt reaction, being long and low, respectively.

It is an object of this invention to provide a continuous method for oxidizing lower alkyl benzene hydrocarbons by the Willgerodt reaction. This continuous method is characterized by high conversions, highyields, and short reaction times.

Pursuant to the invention, a mixture of the alkyl benzene hydrocarbon, water, sulfur and ammonia is passed throughan elongated reaction zone at a rate such thatthe product of the mass velocity of the reaction mixture and the diameter of the reaction zone is above 2000 pounds per foot per hour. The temperature in the reaction zone is maintained in the range 580 F. tov the critical tempera ture of water and the pressure is maintained in the range 2500 to 6000 p. s. i. g.

For most efficient operation of the process of the invention, it is highly desirable that the mixture charged to the reaction zone contain 3 to 4.5 gram atoms of sulfur per alkyl carbon gram atom, 1 to 2 moles of ammonia per mole of alkyl group and to 100 moles of water per mole of hydrocarbon.

When lower alkyl benzene hydrocarbons are oxidized with water, sulfur and ammonia in the manner above described, conversion of the hydrocarbon is essentially 100% and yields of the corresponding benzene carboxylic acid based on the hydrocarbon converted are ordinarily 90 mole per cent or higher.

It is difficult to outline a theory which explains the manner in which the combination of operating variables described above brings about these high conversions and yields. The mechanism of reaction is not well understood and study and observation of the reaction system under reaction conditions are difficult.

It can, however, be stated with assurance that departure from the conditions of reaction above specified is, accompanied by substantial reductions in yield and conversion.

The reaction zone may be generally described as elongated. As a practical matter the reactor is ordinarily a long cylindrical tube or pipe. The length of the reaction zone increases as its cross-sectional area increases, ranging ordinarily from a minimum length of 200. to 300 feet with a 1 inch pipe to 2400 to 3600 feet with a 12 inch pipe. The relationship between the product of the mass velocity of the reaction mixture (in pounds per hour per square foot of cross-sectional area) and the diameter of the reaction zone (in feet) and the yield of benzene carboxylic acids is quite critical. When this product is above about 2000, conversion is essentially complete and the yield based on conversion is of the order of 90. mole per cent. In commercial scale units where the diameter of the reactor will range from about a minimum of about 2 inches to about 12 inches, it is preferred to operate with a mass velocity-diameter product in excess of 10,000. If the cross-section of the reactor tubeis other than circular, then four times the mean hydraulic radius may be used instead of the diameter to calculate the mass velocity-diameter product. While there is no theoreticalmaximum value for the mass velocity-diameter product, about 500,000 constitutes a practical upper limit, since extraordinarily long reaction zones would be called for if operation at higher values were adopted. The pressure drops through such tubes would impose. severe pump requirements and low mechanical efliciency on the system and increase the maintenance burden of the system.

The mole ratio of the reactants does not appear to be critical, excepting, however, the mole ratio of Water to hydrocarbon charge. Ordinarily, sulfur and ammonia are charged in the amounts stoichiometrically required to oxidize the hydrocarbon or in moderate excess over the stoichiometric requirement. No advantage attends the, employment of high molecular excesses of these materials and the presence of such excesses burdens the reaction system with unproductive feed materials. 3 gram atoms of sulfur are stoichiometrically required to oxidize each gram atom of carbon in the alkyl group or groups con.- stituting the side chains of the alkyl benzene hydrocarbon; i. e., if toluene is charged, 3 gram atoms of sulfur should be charged per mole of toluene in order to convertall of the toluene charged. charged, 6 gram atoms of sulfur are stoichiometrically required for each mole of these hydrocarbons. If cumene is charged, 9 gram atoms of sulfur are required per mole, .of cumene.

Alkyl benzenes containing more than 1 carbonv atom in the alkyl groups are oxidized back to the ring so that the ultimate product contains the carboxyl' group directly attached to the nucleus. Since. the alkyl carbon atoms. other than the alpha carbon atom are burned to carbon dioxide, it is preferred to limit the. alkyl benzene hydrocarbon charging stocks to alkyl benzene hydrocarbons containing 1 to 3 carbon atoms in the alkyl groups. While 3 gram atoms of sulfur per gram atom of alkyl carbon meets the stoichiometric requirement, a

moderate excess of sulfur up to about 4.5 gram atoms of sulfur per gram atom of alkyl carbon may be employed. A slight stoichiometric excess of sulfur facilitates the achievement of complete conversion in a minimum reac tion period. 1 mole. of ammonia per alkyl group per mole. of hydrocarbon is stoichiometrically required if. it is desired to convert all of the alkyl benzene to benzene carboxylic acid amides or ammonium salts of, benzene carboxylic acids. A moderate stoichiometric excess of Patented Sept. 11, 1956 If ethyl benzene or a xylene is.

ammonia in the range 1 to 2 moles of ammonia per alkyl group per mole of hydrocarbon may be employed.

The mole ratio of water to hydrocarbon charged to the reaction system must be high, in the range 20:1 to 100:1. If it is attempted to operate with lower. moleratios of water, yields are markedly decreased. Actually, 20 moles of water per mole of hydrocarbon are operable, but with some hydrocarbons, for example, the xylenes, yields and reaction rates are very appreciably better if at least 25 moles of water per mole of xylene are employed and, desirably, at least 30 to 35 moles of water per mole of xylene are employed. As the mole ratio of water to hydrocarbon is increased above 20:1, the yields and reaction rates continue to improve, but a mole ratio of about 100:1 appears to constitute a practical upper limit to this mole ratio. While better yields can be obtained at still higher ratios, the proportion of the reactor space occupied by water becomes so great that the incremental yield does not compensate for the sizing of the reaction zone required for the production of any fixed quantity of benzene, carboxylic acids at the higher mole ratios of water to hydrocarbon.

The temperature in the reaction system must be at least 580 F. if good yields are to be obtained. For example, at 550 F. in a reaction time of 150 minutes only 83.3 mole per cent of isophthalic acid is obtained from meta-xylene, while at 580 F. 90.3 mole per cent yield is obtained in 80 minutes. At 600 F. 90 mole per cent yield is obtained in 45 minutes, and at 630 F. 92.5 mole per cent is obtained in 30 minutes. The temperature of operation, accordingly, must lie in the range 580 F. to the critical temperature of water and, preferably, in the range 600 F. to 675 F.

The reaction tube must be held under a pressure in the range 2500 to 6000 p. s. i. g., the higher pressures being employed at the higher temperatures if good yields are to be obtained.

The residence time of the reaction mixture in the reaction zone should not exceed 75 minutes if losses due to side reactions, decarboxylation and the like, are to be minimized. A residence time in the range -75 minutes and preferably -45 minutes gives adequate time to complete conversion and high yields.

It is believed that when the process is operated under the conditions above described, the reaction mixture passing through the reactor tube is essentially homogeneous, the gas phase being dispersed in the liquid phase and not existing as a distinct continuous phase, and that this condition resulting from the employment of this combination of operating conditions contributes to the efficiency of the process as judged by yield.

Example 1 A mixture of metaand para-xylene containing 85% meta-xylene was passed through a tubular reaction zone having an inside diameter of /1 inch and a length of 400 feet. The mole ratio of xylene to sulfur to ammonia to water was 1:6:2.5:26. During a 4 hour run the product of the mass velocity (pounds per hour per. square foot) and the tube diameter (in feet) was 490. The temperature was 630 F. and the pressure was 3000 p. s. i. 98% conversion of the xylene was obtained. The reaction product was worked up by stripping the reaction product mixture with steam until the pH reached a value of 6 to 7, filtering the stripped reaction product to remove sulfur, and saponifying the filtrate with caustic to decompose isophthalic acid amides and ammonium salts by salts of isophthalic acid. The saponified product was stripped with steam to remove ammonia, acidified to a pH of 6 to 7, and treated with activated. charcoal to remove color bodies. The filtrate from thefiltertreat was acidified to pH 2 with hydrochloric acid to precipitate phthalic acid from the solutions. The phthalic acid s were filtered from the solution and washed with water and dried. The phthalic acids recovered amounted to 70.8 mole per cent of the xylenes converted. p H 1 Example 2 Example 1 was repeated with the exception that during the run the product of the mass velocity and tube diameter was maintained at a value of 5144 during the run. 97.8% of the xylene feed was converted and the yield of phthalic acids based on xylene converted was 92.5 mole per cent.

Example 3 The operation of Example 1 was again repeated with the exception that the product of the mass velocity and tube diameter was maintained at a value of 2310. 97% of the xylene charged was converted and a yield of phthalic acids based on xylenes converted was 91.4 mole per cent.

The above examples indicate the marked improvement in yield which is obtained during operation pursuant to the invention.

While full scale reaction zones employing 2 inch pipe, 6 inch pipe and 12 inch pipe were not constructed and tested, relatively short reaction zones of this diameter were employed to partially oxidize xylenes with water, sulfur and ammonia. While conversions were low and yield had to be judged on the basis of both phthalic acid and toluic acid production, it was evident that the product of mass velocity and reactor diameter must be maintained at at least 2000 pounds per foot per hour if good yields are to be obtained and, further, that with tubes of these larger diameters a considerable advantage is obtained in respect to yield if this product is maintained above 10,000 pounds per foot per hour during the run. The other requirements of the process of the invention, i. e., the temperatures, pressures, residence time and proportions of reactants set out above must be observed irrespective of the diameter of the reaction zone.

It will be appreciated that aqueous ammonium polysulfide may be charged to the reaction instead of sulfur, water and ammonia, both being well recognized as equivalent Willgerodt reagents. The mole ratios of sulfur, water and ammonia to hydrocarbon set out above should be employed whether the oxidizing agent be charged to the reaction in the form of aqueous ammonium polysulfide or as a mixture of water, sulfur and ammonia.

I claim:

1. In a process for oxidizing lower alkyl benzene hydrocarbons by the Willgerodt reaction, the improved method which comprises passing a mixture of the alkyl benzene hydrocarbon, water, sulfur and ammonia containing 3 to 4.5 gram atoms of sulfur per gram atom of alkyl carbon, 1 to 2 moles of ammonia per mole of alkyl group in the alkyl benzene feed and 20 to 100 moles of water per mole of hydrocarbon through an elongated reaction zone at a rate such that the product of the mass velocity of the reaction mixture and the diameter of the reaction zone is in the range 2000 to 500,000 pounds per foot per hour," maintaining in the reaction zone a temperature in the range 580 F. to the critical temperature of water and a pressure in the range 2500 p. s. i. g. to 6000 p. s. i. g. and maintaining the reaction mixture in residence in the reaction zone for a period not in excess of 75 minutes.

2. A process as defined in claim 1, wherein the alkyl benzene hydrocarbon is a xylene.

3. In a process for oxidizing lower alkyl benzene hydrocarbons by the Willgerodt reaction, the improved method which comprises continuously passing a mixture of the alkyl benzene hydrocarbon, water, sulfur and ammonia containing 3 to 4.5 gram atoms of sulfur per gram atom of alkyl carbon, 1 to 2 moles of ammoniarper mole of alkyl group in the alkyl benzene feed and 20 to 100 moles of water per mole of hydrocarbon through an elongated reaction zone at a rate such that the product of the mass velocity of the reaction mixture and the diameter of the reaction zone is in the range 2000 to 500,000 pounds per foot per hour, maintaining inthe reaction zonea temperature in the range of 580. F. to. the critiqal temperature of water and a pressure in the, range 2500 p, s..i. g..to

6000 p. s. i. g. and maintaining the reaction mixture in residence in the reaction zone for a period of about 10 to 75 minutes.

4. In a process for oxidizing xylenes by the Willgerodt reaction, the improved method which comprises continuously passing a mixture of xylene, water, sulfur and ammonia containing about 6 to 9 gram atoms of sulfur per mole of xylene, about 2 to 4 moles of ammonia per mole of xylene and about 20 to 100 moles of water per mole of xylene through an elongated reaction zone at a rate such that the product of the mass velocity of the reaction mixture and the diameter of the reaction zone is in excess of 2000 pounds per foot per hour, maintaining in the reaction zone a temperature in the range 580 F. to the critical temperature of Water and a pressure in the range 2500 p. s. i. g. to 6000 p. s. i. g. and maintaining the reaction mixture in residence in the reaction zone for a period not in excess of 75 minutes.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN A PROCESS FOR OXIDIZING LOWER ALKYL BENZENE HYDROCARBONS BY THE WILLGERODT REACTION, THE IMPROVED METHOD WHICH COMPRISES PASSING A MIXTURE OF THE ALKYL BENZENE HYDROCARBON, WATER, SULFUR AND AMMONIA CONTAINING 3 TO 4.5 GRAM ATOMS OF SULFUR PER GRAM ATOM OF ALKYL CARBON, 1 TO 2 MOLES OF AMMONIA PER MOLE OF ALKYL GROUP IN THE ALKYL BENZENE FEED AND 20 TO 100 MOLES OF WATER PER MOLE OF HYDROCARBON THROUGH AN ELONGATED REACTION ZONE AT A RATE SUCH THAT THE PRODUCT OF THE MASS VELOCITY OF THE REACTION MIXTURE AND THE DIAMETER OF THE REACTION ZONE IS IN THE RANGE 2000 TO 500,000 POUNDS PER FOOT PER HOUR, MAINTAINING IN THE REACTION ZONE A TEMPERATURE IN THE RANGE 580*F. TO THE CRITICAL TEMPERATURE OF WATER AND A PRESSURE IN THE RANGE 2500 P. S. I. G. TO 6000 P. S. I. G. AND MAINTIANING THE REACTION MIXTURE IN RESIDENCE IN THE REACTION ZONE FOR A PERIOD NOT IN EXCESS OF 75 MINUTES. 